An oscillometric method and a tonometry method have been conventionally known as a blood pressure measurement method. The oscillometric method is to measure blood pressure by, in a process of putting a cuff on an upper arm or wrist to compress a blood vessel and to make a blood flow therein once stop and then loosing the cuff to gradually reduce the pressure, and checking a cuff pressure which reflects pulsation of a blood vessel wall synchronizing with cardiac pulsation. Blood pressure measurement by the oscillometric method using a cuff is non-invasive, and has been automatically performed in a mechanical manner, for easy blood pressure measurement. However, the blood pressure measurement device is too large to carry around, takes several ten seconds to one minute to perform measurement, and requires resting during blood pressure measurement, and thus, the frequency of blood pressure measurement disadvantageously interferes with daily life.
The tonometry method is a method for obtaining a blood pressure value while pressing a sensor having a flat contact pressure against an artery, and measuring variation in inner pressure in the artery pulsating against the sensor. A conceptual diagram of blood pressure measurement by the tonometry method is illustrated in FIG. 11. FIG. 11 is a schematic cross-sectional view of a portion of and around a human hand, where a radial artery is positioned on a radius bone, and a sensor array is put on a skin immediately above the radial artery. The sensor array generally has a flat shape, so that upon pressing the sensor array against the skin, the skin facing a sensor contact surface is also expanded flat.
In the sensor array, a plurality of sensors are arranged in an array as the name suggests, and a sensor positioned around a portion immediately above the radial artery can capture an arterial pulse wave in the largest dynamic range. Therefore, if a sensor array has a size not less than a certain size, it may be put on a roughly estimated position of a blood vessel to adopt a signal of the largest dynamic range among signals obtained from respective sensors.